1. Field of the Invention
The present invention relates to a compact head-up display having a large exit pupil also sometimes referred to as a head-up viewer, head-up collimator or head-up visualization system. More particularly, the present invention relates to such a display having a decreased power consumption.
2. Description of the Related Art
Head-up displays, also known as HUDs, are augmented reality display systems which enable to integrate visual information on a real scene seen by an observer. In practice, such systems may be placed in a helmet visor, in the cockpit of a plane, or in the interior of a vehicle. They are thus positioned at a short distance from the user's eyes, for example, a few centimeters or tens of centimeters away from them.
FIG. 1 schematically illustrates the operation of such a device.
A beam splitter 10 is placed between the eye of user 12 and a scene to be observed 14. The objects of the scene to be observed are generally located at infinity or at a long distance from the observer. Beam splitter 10 is placed according to a 45° angle relative to the axis between scene 14 and observer 12 to transmit the information originating from scene 14 to observer 12, without altering this information.
To project an image seen at the same distance as the real image of the scene and to overlay it thereon, a projection system is provided. This system comprises an image display element 16, for example, a screen, located at the object focal point of an optical system 18. The image displayed on the screen is thus collimated to infinity by optical system 18. The user does not have to make any effort of accommodation, which limits his/her visual fatigue.
The projection system is placed perpendicularly to the axis between the scene and the observer so that the beam originating from optical system 18 reaches beam splitter 10 perpendicularly to this axis. The beam originating from optical system 18 thus reaches beam splitter 10 with a 45° angle relative to its surface.
Beam splitter 10 combines the image of scene 14 and the image originating from projection system 16-18, whereby observer 12 visualizes an image comprising the projected image overlaid on the image of scene 14.
To visualize the image projected by projection system 16-18, the observer's eye should be placed in the area of reflection of the beam originating from optical system 18 on splitter 10. An important constraint to be respected is to take into account the possible motions of the user's head in front of the projector, and thus to provide the largest possible beam at the exit of optical system 18. In other words, an optical system 18 having a large exit pupil, for example in the range from a few centimeters to a few tens of centimeters, should be provided, so that the observer's head motions do not imply a loss of the projected information.
Another constraint of head-up systems is to provide a relatively compact device. Indeed, significant bulk constraints bear on these devices, particularly when they are used in plane cockpits or in the interior of vehicles of limited volume. To limit the bulk of head-up displays, devices having a decreased focal distance should thus be provided.
It is thus desired to obtain devices having a very small exit aperture, that is, the ratio of the object focal distance of the system to the diameter of the exit pupil of the device. The complexity of an optical system is known to depend on the exit aperture thereof. More particularly, the smaller the aperture of a device, the more complex the device. The more complex the optical system, the larger the number of optical elements that it contains, particularly to limit the different geometric aberrations. This increase in the number of elementary optical elements increases the volume and the cost of the complete device, which is not desired.
It is further necessary to provide devices having a low power consumption.